Aerotech G-Force Ejection charge question.

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Race58

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I have built an Aerotech G-Force to use for Mid Power. This is my first venture into Mid Power so I have read and asked questions about it.
I decided to upgrade a little so I replaced the coupler with a longer and thicker (8") one from GLR using the existing one to beef up the rear tube aft of the motor. I also swapped out the composite forward bulkhead and screw eye for a 3/32 ply and u-bolt. I went with 12' of 1/4" Tublar kevlar,5' of 9/16 Tubular nylon and a small piece of the original 5/8" elastic attached to a 48" Sperachute with a kevlar chute protector. All of this was connected with quick links. I also added removable rivets to the coupler/lower body tube and to the nosecone to upper body tube to be able to remove them later when I add a camera.

In doing this I figured that I should ground test the ejection charge to see if the upper section and the laundry will deploy properly.

That's where I found a problem. I tried the test by loading the charge for a "G" motor into the fwd closure of my 29mm case along with an ignitor.
When the ejection charge fired the upper section it only moved up the coupler about 2-3 inches. See video here:


I tried packing things inside tighter to make them more loose (even though they were already loose enough I thought)and tried again with same result.
The upper section is not too tight on the coupler in my estimation. When I pick up the rocket by the upper section the bottom slides easy off and when I turn it nose down it stays put.

Maybe I'm missing something so I'm asking here. I am glad that I did this first because they way it seems that if I had loaded the motor and just flew it:surprised: probably would not have ended well.

I have searched but could not find any reference to mid power ground test's only high power. Not that they aren't there I just could find any. Probably the same and seems that adding more BP to the ejection does the trick. I could try that but what about the SU motors? You can't change them.

In looking at the video I noticed a lot of ejection and flame thru the aft closure. Could the lack of back pressure (Empty casing) be the problem?

Thanks for any help.
 
I have built an Aerotech G-Force to use for Mid Power. This is my first venture into Mid Power so I have read and asked questions about it.
I decided to upgrade a little so I replaced the coupler with a longer and thicker (8") one from GLR using the existing one to beef up the rear tube aft of the motor. I also swapped out the composite forward bulkhead and screw eye for a 3/32 ply and u-bolt. I went with 12' of 1/4" Tublar kevlar,5' of 9/16 Tubular nylon and a small piece of the original 5/8" elastic attached to a 48" Sperachute with a kevlar chute protector. All of this was connected with quick links. I also added removable rivets to the coupler/lower body tube and to the nosecone to upper body tube to be able to remove them later when I add a camera.

In doing this I figured that I should ground test the ejection charge to see if the upper section and the laundry will deploy properly.

That's where I found a problem. I tried the test by loading the charge for a "G" motor into the fwd closure of my 29mm case along with an ignitor.
When the ejection charge fired the upper section it only moved up the coupler about 2-3 inches. See video here:


I tried packing things inside tighter to make them more loose (even though they were already loose enough I thought)and tried again with same result.
The upper section is not too tight on the coupler in my estimation. When I pick up the rocket by the upper section the bottom slides easy off and when I turn it nose down it stays put.

Maybe I'm missing something so I'm asking here. I am glad that I did this first because they way it seems that if I had loaded the motor and just flew it:surprised: probably would not have ended well.

I have searched but could not find any reference to mid power ground test's only high power. Not that they aren't there I just could find any. Probably the same and seems that adding more BP to the ejection does the trick. I could try that but what about the SU motors? You can't change them.

In looking at the video I noticed a lot of ejection and flame thru the aft closure. Could the lack of back pressure (Empty casing) be the problem?

Thanks for any help.

I see the flash out the aft end, I'm guessing it is tossing your casing out rather than tossing your laundry out. Retain the casing and try again, I'd be willing to bet it works.
 
in a fully loaded motor, there is a nozzle in the aft end to restrict the leakage out the aft end. I also saw the huge flash in the video and wondered if the casing was blown out the back end or if you even used a casing (did you ONLY use a forward closure by itself???).

Also, after normal motor burn, there is some crud inside the casing and in the delay housing to also restrict the leakage out the aft end.
 
No, I did not just load the fwd closure. What I did was to follow directions given to me from Aerotech via email.
they are as follows:

"Take your reloadable case, DO NOT LOAD IT, run an igniter through the hole in the forward closure (were the ejection charge goes) so the igniter is in the ejection charge well, and the wires are hanging out the Aft closure. Put your ejection charge in the forward closure as you would to fly it. Place the motor in the rocket, SECURE IT in, load your recovery system as it would fly, get back, use your launch controller and test fire it."

I took some scrap plywood and made PMR for it and secured it like this. The case did not move.

IMG_1302.jpg


As I said before I thought that there was possibly not enough back pressure to force the ejection charge forward.
If that is the case how do you test it (or can you?) without loading the case and running a full engine test after securing the rocket from moving?
 
I've flown my G-Force a few times, and have never had a problem with separation. I have had problems with the chute not opening. The G-Force chute is very light-weight, and it will stayed balled up if the shroud lines are wrapped around it. Since you are using a different chute you shouldn't have that problem.

I ran a quick test on my G-Force by blowing on the end of it, and it separated without requiring much force. Does air flow freely through your ejection baffle? Maybe it's clogged up. The G-Force shouldn't require much pressure to separate. With a 4-inch body tube, a pressure of just 2 PSI would provide 25 pounds of force.

Dave
 
Well, it might just be the lack of crud in the delay well and the lack of nozzle - but the tiny hole in the ejection well and the igniter wires should have blocked a decent amount of the aft blowk-back. Of course the video showed a lot blowing out the back...

If you are still worried, simply sand down the coupler until it slides smoother and looser. Then there will be no problem.
 
Fred,,

Nah, Not worried just being cautious. I think as I suspected and you and others seem to confirm that there just was not enough restriction aft and the ejection charge took the "path of least resistance".
I think it will be fine so I'll try and launch later this week and we'll see.:D


Thanks to all.
Ed
 
One thing you can do is cut another piece of plywood similar to the retention block you already have, but don't cut anything out in the middle for the motor. Then you could use this piece of plywood as a seal as well as rentention.

Frankly, considering how the model reacted in the video, I would be afraid the coupler is too tight. Some people to say it should be tight enough to be able to hold the rocket by the nose without the rocket coming apart. But that does not mean it is not too tight. I like to hold it by the nose and then shake it a little. It should be loose enough to start coming off if you do that.
 
I don't think that the coupler is the problem. As I said in my original post to start this thread I can lift the rocket by it's nose and the bottom section slides off. When I hold it upside down the upper section stays put. I actually think that it is too loose.

As far as blocking the bottom as you said I think that might work to keep the pressure going forward instead of aft.
One thing you can do is cut another piece of plywood similar to the retention block you already have, but don't cut anything out in the middle for the motor. Then you could use this piece of plywood as a seal as well as rentention.

Frankly, considering how the model reacted in the video, I would be afraid the coupler is too tight. Some people to say it should be tight enough to be able to hold the rocket by the nose without the rocket coming apart. But that does not mean it is not too tight. I like to hold it by the nose and then shake it a little. It should be loose enough to start coming off if you do that.
 
If you are concerned you can add some extra black powder to the cap since it is not fully filled.
 
You know, there might be another reason for what happened. Did you seal the hole in the forward enclosure after sticking in the ignitor? Maybe some of the ejection charge dribbled out of the ejection charge well and into the case proper.
 
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